If you are obscenely low on battery, connected to WiFi, just need your phone for iMessage, and general app this tip is for you. If you can sacrifice receiving or making phone calls you can go in Airplane Mode with WiFi enabled. This is my method for extreme battery savings. I use this when I know I can’t take a phone call but still need connectivity.
Remember, this only works if you can connect to a WiFi network and when you simply need to use your phone as everything but a phone.
Simply swipe up on the Control Center, turn on Airplane Mode by pressing the airplane icon and then after a moment pressing the WiFi button. You’ll connect back to your WiFi network and be able to use apps that don’t need cell phone service.